You should be watching the slack in the line after you "pop"...
If the line does ANYTHING unusual while returning to it's original state..then set the hook....crappie tend to bite on the fall of the jig....sometimes he will just come up from under the jig and inhale it and not move..this will show in your line as an unusual stopping of the fall of the jig or a wierd sideways movement of the jig...
You won't always feel the bite..so watch that line as the jig is falling for unusual movements...
Sometimes you want that slower fall of a lighter jig....